It is well-known fact that there are various methods of coating various paint films on a flexible support which runs, such as roll coating, gravure coating, extrusion coating, slide bead coating and curtain coating.
A magnetic recording medium is obtained by, for example, coating and drying a magnetic paint solution on a support. Coating methods generally used for this purpose include roll coating, gravure coating and extrusion coating. Of these methods, extrusion coating is superior to the others since it provides uniform coat thickness.
On the other hand, with the recent rapid progress of improvement of magnetic recording media themselves, there have been increasing demands for use of magnetic powder of oxides with high BET values and barium ferrite materials, and the viscosity of coating solution has tended to increase.
There have also been increasing demands for high speed coating for increased productivity.
Also, recent trends in magnetic recording media have involved the formation of an under coat layer and/or interlayer, the formation of magnetic layer thereon, and multiplication of magnetic layer.
By the way, known conventional methods of extrusion coating mainly for the purpose of production of magnetic recording media include those described in Japanese Patent Publication Open to Public Inspection Nos. 84771/1982, 104666/1983 and 238179/1985.
Known prior art of multiple-layer coating includes the methods described in Japanese Patent Publication Open to Public Inspection Nos. 164022/1988, 164023/1988, 214524/1987, 124631/1987 and 92132/1987 and Japanese Patent Examined Publication No. 17291/1982.
The present inventors previously proposed in Japanese Patent Application No. 34859/1988 an extrusion type coating head comprising an apparatus which satisfies the demands described above and which continuously extrudes a coating solution onto the surface of a flexible support which runs continuously along the front and back edge faces via the slit between the front and back edge faces and applies the coating solution on the face of the support, wherein the end portion of said front edge has an arc face with a curvature radius of r.sub.1 to the end tip, wherein the entire face of said back edge is an arc face with a curvature radius of r.sub.2, wherein these curvature radii satisfy the requirements specified by the following formulas: EQU 3 mm&lt;r.sub.2 &lt;20 mm EQU 1 mm&lt;r.sub.1 &lt;r.sub.2 /2 mm
and wherein at least a portion of the back edge face is projected beyond the tangent line at the end tip of the front edge face.
This extrusion type coating head is schematized in FIG. 1A and FIG. 1B.
This extrusion type coating head, when used to coat a single layer, is advantageous in that neither uneven coating nor streaking occurs and that excellent coating performance is obtained even under high speed coating conditions.
However, when using two units of this head for so called wet (sequential) multiple-layer coating, in which an upper layer is formed while the previously coated lower layer remains in a wet state, the lower layer is scraped off during upper layer coating and thus it is difficult to coat the upper layer stably in cases where the viscosity of the magnetic solution for the lower layer is lower than the coating solution for the upper layer.
In addition, this tendency occurs not only in the use of this head is used but also it is normally noted in cases where an extrusion type coating head for single layer coating is used.
Taking note of this situation, the present inventors proposed in the specification for a recent patent application (Japanese patent application filed on Mar. 20, 1989 under the title of COATING APPARATUS) an invention which permits simultaneous coating (so called wet-on-wet multiple-layer coating) of a lower layer and an upper layer on a support in a wet state by means of a single extrusion type coating head, the invention comprising "a coating apparatus which continuously extrudes a coating solution onto the surface of a flexible support which runs sequentially and continuously along the front, center and back edge faces via each of the first slit between the front and center edge faces and the second slit between the center and back edge faces and applies the coating solution on the face of the support, wherein the end portion of said front edge has at least a linear portion along the running direction, wherein said linear portion has a length of not more than 1 mm, and wherein at least a portion of the center and back edge faces is projected toward the support side beyond the line extended from the linear portion".
This coating apparatus is schematized in FIG. 2.
This coating apparatus permits simultaneous coating of a lower layer and an upper layer on a support even when the viscosity of the lower layer coating solution is lower than that of the upper layer coating solution, but it is hard to say that multiple-layer coating can be achieved without any hamper even in cases where the paint film of the lower layer is sufficiently thin.
With this background, the present inventors made various investigations of methods which permit wet multiple-layer coating without any hamper even in cases where the viscosity of the lower layer coating solution is lower than that of the upper layer coating solution and where the paint film of the lower layer is sufficiently thin, and found that this purpose can be accomplished by use of an extrusion type coating apparatus as described below. The present inventors made further investigations based on this finding, and developed the present invention.